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existence than the present time. Whatever the causes may have been, we
can see that certain districts and times would have been much more
favorable than others for the development of so large a quadruped as
the giraffe.

In order that an animal should acquire some structure specially and
largely developed, it is almost indispensable that several other parts should
be modified and coadapted. Although every part of the body varies slightly,
it does not follow that the necessary parts should always vary in the right
direction and to the right degree. With the different species of our domesti-
cated animals we know that the parts vary in a different manner and
degree, and that some species are much more variable than others. Even if
the fitting variations did arise, it does not follow that natural selection
would be able to act on them and produce a structure which apparently
would be beneficial to the species. For instance, if the number of individuals
existing in a country is determined chiefly through destruction by beasts of
prey — by external or internal parasites, etc. — as seems often to be the case,


then natural selection will be able to do little, or will be greatly retarded,
in modifying any particular structure for obtaining food. Lastly, natural
selection is a slow process, and the same favorable conditions must long
endure in order that any marked effect should thus be produced. Except
by assigning such general and vague reasons, we cannot explain why, in
many quarters of the world, hoofed quadrupeds have not acquired mudi
elongated necks or other means for browsing on the higher branches
of trees.

Objections of the same nature as the foregoing have been advanced by
many writers. In each case various causes, besides the general ones just
indicated, have probably interfered with the acquisition through natural
selection of structures, which it is thought would be beneficial to certain
species. One writer asks, why has not the ostrich acquired the power of
flight? But a moment's reflection will show what an enormous supply of
food would be necessary to give to this bird of the desert force to move its >'
huge body through the air. Oceanic islands are inhabited by bats and seals, i
but by no terrestrial mammals; yet as some of these bats are peculiar'
species, they must have long inhabited their present homes. Therefore Sir '
C. LyeU asks, and assigns certain reasons in answer, why have not seals and.
bats given birth on such islands to forms fitted to live on the land? but seals,
would necessarily be first converted into terrestrial carnivorous animals of
considerable size, and bats into terrestrial insectivorous animals; for the
former there would be no prey; for the bats ground -insects would serve as
food, but these would already be largely preyed on by the reptiles or birds,
which first colonize and abound on most oceanic islands. Gradations of
structure, with each stage beneficial to a changing species, will be favored
only under certain peculiar conditions. A strictly terrestrial animal, by
occasionally hunting for food in shallow water, then in streams or lakes,
might at last be converted into an animal so thoroughly aquatic as to brave
the open ocean. But seals would not find on oceanic islands the conditions
favorable to their gradual reconversion into a terrestrial form. Bats, as
formerly shown, probably acquired their wings by at first gliding through
the air from tree to tree, like the so-called flying squirrels, for the sake of
escaping from their enemies, or for avoiding falls; but when the power of
true flight had once been acquired, it would never be reconverted back, at
least for the above purposes, into the less eflicient power of gliding through
the air. Bats might, indeed, like many birds, have had their wings greatly
reduced in size, or completely lost, through disuse; but in this case it would
be necessary that they should first have acquired the power of running
quickly on the ground, by the aid of their hind legs alone, so as to compete
with birds or other ground animals: and for such a change a bat seems
singularly ill-fitted. These conjectural remarks have been made merely to
show that a transition of structure, with each step beneficial. Is a highly
complex affair; and that there is nothing strange In a transition not having
occurred In any particular case.

Lastly, more than one writer has asked why have some animals had


their mental powers more highly developed than others, as such develop-
ment would be advantageous to all? Why have not apes acquired the in-
tellectual powers of man? Various causes could be assigned; but as they
jare conjectural, and their relative probability cannot be weighed, it would
be useless to give them. A definite answer to the latter question ought not
to be expected, seeing that no one can solve the simpler problem, why, of
two races of savages, one has risen higher in the scale of civilization than
the other; and this apparently implies increased brain power.

We will return to Mr. Mivart's other objections. Insects often resemble,
for the sake of protection, various objects, such as green or decayed leaves,
dead twigs, bits of lichen, flowers, spines, excrement of birds, and living
insects; but to this latter point I shall hereafter recur. The resemblance is
often wonderfully close, and is not confined to color, but extends to form,
and even to the manner in which the insects hold themselves. The cater-
pillars which project motionless like dead twigs from the bushes on which
they feed, offer an excellent instance of a resemblance of this kind. The
cases of the imitation of such objects as the excrement of birds, are rare and
exceptional. On this head, Mr. Mivart remarks, "As, according to Mr. Dar-
win's theory, there is a constant tendency to indefinite variation, and as the
minute incipient variations will be in all directions, they must tend to
neutralize each other, and at first to form such unstable modifications that
it is difficult, if not impossible, to see how such indefinite oscillation, of in-
finitesimal beginnings can ever build up a sufficiently appreciable resem-
blance to a leaf, bamboo, or other object, for natural selection to seize upon
and perpetuate."

But in all the foregoing cases the insects in their original state no doubt
presented some rude and accidental resemblance to an object commonly
found in the stations frequented by them. Nor is this at all improbable,
considering the almost infinite number of surrounding objects and the
diversity in form and color of the hosts of insects which exist. As some rude
resemblance is necessary for the first start, we can understand how it is
that the larger and higher animals do not (with the exception, as far as I
know, of one fish) resemble for the sake of protection special objects, but
only the surface which commonly surrounds them, and this chiefly in color.
Assuming that an insect originally happened to resemble in some degree a
dead twig or a decayed leaf, and that it varied slightly in many ways, then
all the variations which rendered the insect at all more like any such object,
and thus favored its escape, would be preserved, while other variations
would be neglected and ultimately lost; or, if they rendered the insect at all
less like the imitated object, they would be eliminated. There would indeed
be force in Mr. Mivart's objection, if we were to attempt to account for
the above resemblances, independently of natural selection, through mere
fluctuating variability; but as the case stands there is none.

Nor can I see any force in Mr. Mivart's difficulty with respect to "the
last touches of perfection in the mimicry"; as in the case given by Mr.
Wallace, of a walking-stick insect (Ceroxylus laceratus), which resembles


"a stick grown over by a creeping moss or jungermannia." So close was tin
resemblance, that a native Dyak maintained that the foliaceous excrescence
were really moss. Insects are preyed on by birds and other enemies whos
sight is probably sharper than ours, and every grade in resemblance whici
aided an insect to escape notice or detection, would tend toward its pres
ervation; and the more perfect the resemblance, so much the better fo
the insect. Considering the nature of the differences between the species i:
the group which includes the above Ceroxylus, there is nothing improbabl
in this insect having varied in the irregularities on its surface, and in thes
having become more or less green-colored; for in every group the charac
ters which differ in the several species are the most apt to vary, while tb
generic characters, or those common to all the species, are the most constant

The Greenland whale is one of the most wonderful animals in the world
and the baleen, or whalebone, one of its greatest peculiarities. The baleei
consists of a row, on each side of the upper jaw, of about 300 plates o.
laminse, which stand close together transversely to the longer axis of the
mouth. Within the main row there are some subsidiary rows. The extremitie
and inner margins of all the plates are frayed into stiff bristles, which clotb
the whole gigantic palate, and serve to strain or sift the water, and thus t(
secure the minute prey on which these great animals subsist. The middk
and longest lamina in the Greenland whale is ten, twelve, or even fifteer
feet in length; but in the different species of Cetaceans there are gradation!
in length; the middle lamina being in one species, according to Scoresby
four feet, in another three, in another eighteen inches, and in the Balaenop-
tera rostrata only about nine inches in length. The quality of the whalebone
also differs in the different species.

With respect to the baleen, Mr. Mivart remarks that if it "had once
attained such a size and development as to be at all useful, then its preserva-
tion and augmentation within serviceable limits would be promoted by
natural selection alone. But how to obtain the beginning of such useful de-
velopment?" In answer, it may be asked, why should not the early progeni-
tors of the whales with baleen have possessed a mouth constructed some-
thing like the lamellated beak of a duck? Ducks, like whales, subsist by
sifting the mud and w^ater; and the family has sometimes been called
Crihlatores, or sifters. I hope that I may not be misconstrued into saying
that the progenitors of whales did actually possess mouths lamellated like the
beak of a duck. I wish only to show that this is not incredible, and that the
immense plates of baleen in the Greenland whale might have been de-
veloped from such lamellae by finely graduated steps, each of service to its'

The beak of the shoveller-duck (Spatula clypeata) is a more beautiful
and complex structure than the mouth of the whale. The upper mandible:
is furnished on each side (in the specimen examined by me) with a row
or comb formed of 188 thin, elastic lamellse, obliquely beveled so as to be
pointed, and placed transversely to the longer axis of the mouth. They


* arise from the palate, and are attached by flexible membrane to the sides
of the mandible. Those standing toward the middle are the longest, being
about one-third of an inch in length, and they project fourteen one-
hundredths of an inch beneath the edge. At their bases there is a short
subsidiary row of obliquely transverse lamellae. In these several respects

' they resemble the plates of baleen in the mouth of a whale. But toward
the extremity of the beak they differ much, as they project inward, instead
of straight downward. The entire head of the shoveller, though incompa-
rably less bulky, is about one-eighteenth of the length of the head of a
moderately large Bal^noptera rostrata, in which species the baleen is
only nine inches long; so that if we were to make the head of the shoveller

' as long as that of the Balasnoptera, the lamellae would be six inches in
length, that is, two-thirds of the length of the baleen in this species of
whale. The lower mandible of the shoveller-duck is furnished with lamellae
of equal length with these above, but finer; and in being thus furnished
it differs conspicuously from the lower jaw of a whale, which is destitute
of baleen. On the other hand, the extremities of these lower lamellse are
frayed into fine bristly points, so that they thus curiously resemble the
plates of baleen. In the genus Prion, a member of the distinct family of
the Petrels, the upper mandible alone is furnished with lamellae, which
are well developed and project beneath the margin; so that the beak of
this bird resembles in this respect the mouth of a whale.

From the highly developed structure of the shoveller's beak we may
proceed (as I have learned from informiation and specimens sent to me
by Mr. Salvin), without any great break, as far as fitness for sifting is
concerned, through the beak of the Merganetta armata, and in some
respects through that of the Aix sponsa, to the beak of the common duck.
In this latter species the lamella are much coarser than in the shoveller, and
are firmly attached to the sides of the mandible; they are only about fifty
in number on each side, and do not project at all beneath the margin.
They are square-topped, and are edged with translucent, hardish tissue,
as if for crushing food. The edges of the lower mandible are crossed by
numerous fine ridges, which project very little. Although the beak is thus
very inferior as a sifter to that of a shoveller, yet this bird, as every one
knows, constantly uses it for this purpose. There are other species, as I
hear from Mr. Salvin, in w^hich the lamellae are considerably less developed
than in the common duck; but I do not know \vhether they use their beaks
for sifting the water.

Turning to another group of the same family. In the Eg\^tian goose
(Chenalopex) the beak closely resembles that of the common duck; but
the lamellae are not so numerous, nor so distinct from each other, nor do
they project so much imvard; yet this goose, as I am informed by Mr. E.
Bartlett, "uses its bill like a duck by throwing the water out at the corners."
Its chief food, however, is grass, which it crops like the common goose. In
this latter bird the lamellae of the upper mandible are much coarser than
in the common duck, almost confluent, about twenty-seven in number on


each side, and terminating upward in teeth-like knobs. The palate is also
covered with hard rounded knobs. The edges of the lower mandible are
serrated with teeth much more prominent, coarser, and sharper than in
the duck. The common goose does not sift the water, but uses its beak
exclusively for tearing or cutting herbage, for which purpose it is so well
fitted that it can crop grass closer than almost any other animal. There
are other species of geese, as I hear from Mr. Bartlett, in which the lamellse
are less developed than in the common goose.

We thus see that a member of the duck family, with a beak constructed
like that of a common goose and adapted solely for grazing, or even a
member with a beak having less well developed lamellae, might be con-
verted by small changes into a species like the Egyptian goose — this into
one like the common duck — and, lastly, into one like the shoveller, pro-
vided with a beak almost exclusively adapted for sifting the water; for
this bird could hardly use any part of its beak, except the hooked tip, for
seizing or tearing solid food. The beak of a goose, as I may add, might
also be converted by small changes into one provided with prominent,
recurved teeth, like those of the Merganser (a member of the same family),
serving for the widely different purpose of securing live fish.

Returning to the whales. The Hyperoodon bidens is destitute of true
teeth in an efficient condition, but its palate is roughened, according to
Lacepede, with small, unequal, hard points of horn. There is, therefore,
nothing improbable in supposing that some early Cetacean form was pro-
vided with similar points of horn on the palate, but rather more regularly
placed, and which, like the knobs on the beak of the goose, aiding it in
seizing or tearing its food. If so, it will hardly be denied that the points
might have been converted through variation and natural selection into
lamellae as well developed as those of the Egyptian goose, in which case
they would have been used both for seizing objects and for sifting the water;
then into lamellae like those of the domestic duck; and so onward, until
they became as well constructed as those of the shoveller, in which case
they would have served exclusively as a sifting apparatus. From this
stage, in which the lamellae would be two-thirds of the length of the plates
of baleen in the Balaenoptera rostrata, gradations, which may be observed
in still-existing Cetaceans, lead us onward to the enormous plates of baleen
in the Greenland whale. Nor is there the least reason to doubt that each
step in this scale might have been as serviceable to certain ancient Cetaceans,
with the functions of the parts slowly changing during the progress of
development, as are the gradations in the beaks of the different existing
members of the duck-family. We should bear in mind that each species
of duck is subjected to a severe struggle for existence, and that the struc-
ture of every part of its frame must be well adapted to its conditions of life.

The Pleuronectidae, or Flat-fish, are remarkable for their asymmetrical
bodies. They rest on one side — in the greater number of species on the
left, but in some on the right side ; and occasionally reversed adult specimens
occur. The lower, or resting-surface, resembles at first sight the ventral


surface of an ordinary fish; it is of a white color, less developed in many
ways than the upper side, with the lateral fins often of smaller size. But the
eyes offer the most remarkable peculiarity; for they are both placed on
the upper side of the head. During early youth, however, they stand op-
posite to each other, and the whole body is then symmetrical, with both
sides equally colored. Soon the eye proper to the lower side begins to glide
slowly round the head to the upper side; but does not pass right through
the skull, as was formerly thought to be the case. It is obvious that unless
the lower eye did thus travel round, it could not be used by the fish while
lying in its habitual position on one side. The lower eye would, also, have
been liable to be abraded by the sandy bottom. That the Pleuronectidae
are admirably adapted by their flattened and asymmetrical structure for
their habits of life, is manifest from several species, such as soles, flounders,
etc., being extremely common. The chief advantages thus gained seem to
be protection from their enemies, and facility for feeding on the ground.
The different members, however, of the family present, as Schiodte remarks,
"a long series of forms exhibiting a gradual transition from Hippoglossus
pinguis, which does not in any considerable degree alter the shape in which
it leaves the ovum, to the soles, which are entirely thrown to one side."

Mr. Mivart has taken up this case, and remarks that a sudden spontane-
ous transformation in the position of the eyes is hardly conceivable, in
which I quite agree with him. He then adds: "If the transit was gradual,
then how such transit of one eye a minute fraction of the journey toward
the other side of the head could benefit the individual is, indeed, far from
cleatr. It seems, even, that such an incipient transformation must rather
have been injurious." But he might have found an answer to this objection
in the excellent observations published in 1867 by Malm. The Pleu-
ronectidae, while very young and still symmetrical, with their eyes stand-
ing on opposite sides of the head, cannot long retain a vertical position,
owing to the excessive depth of their bodies, the small size of their lateral
fins, and to their being destitute of a swim-bladder. Hence, soon growing
tired, they fall to the bottom on one side. While thus at rest they often
twist, as Malm observed, the lower eye upward, to see above them; and
they do this so vigorously that the eye is pressed hard against the upper
part of the orbit. The forehead between the eyes consequently becomes,
as could be plainly seen, temporarily contracted in breadth. On one occasion
Malm saw a young fish raise and depress the lower eye through an angular
distance of about seventy degrees.

We should remember that the skull at this early age is cartilaginous
and flexible, so that it readily yields to muscular action. It is also known
with the higher animals, even after early youth, that the skull yields and
is altered in shape, if the skin or muscles be permanently contracted through
disease or some accident. With long-eared rabbits, if one ear flops forward
and downward, its weight drags forward all the bones of the skull on the
same side, of which I have given a figure. Malm states that the newly-
hatched young of perches, salmon, and several other symmetrical fishes,


have the habit of occasionally resting on one side at the bottom; and he
has observed that they often then strain their lower eyes so as to look
upward; and their skulls are thus rendered rather crooked. These fishes,
however, are soon able to hold themselves in a vertical position, and no t
permanent effect is thus produced. With the Pleuronectidae, on the other
hand, the older they grow the more habitually they rest on one side, owing
to the increasing flatness of their bodies, and a permanent effect is thus
produced on the form of the head, and on the position of the eyes. Judging
from analogy, the tendency to distortion would no doubt be increased
through the principle of inheritance. Schiodte believes, in opposition to
some other naturalists, that the Pleuronectidae are not quite symmetrical
even in the embryo; and if this be so, we could understand how it is
that certain species, while young, habitually fall over and rest on the left
side, and other species on the right side. Malm adds, in confirmation of
the above view, that the adult Trachypterus arcticus, which is not a
member of the Pleuronectidae, rests on its left side at the bottom, and
swims diagonally through the water; and in this fish, the two sides of the
head are said to be somewhat dissimilar. Our great authority on Fishes,
Dr. Giinther, concludes his abstract of Malm's paper, by remarking that
"the author gives a very simple explanation of the abnormal condition of
the Pleuronectoids."

We thus see that the first stages of the transit of the eye from one side
of the head to the other, which Mr. Mivart considers would be injurious,
may be attributed to the habit, no doubt beneficial to the individual and
to the species, of endeavoring to look upward with both eyes, while rest-
ing on one side at the bottom. We may also attribute to the inherited effects
of use the fact of the mouth in several kinds of flat-fish being bent to-
ward the lower surface, with the jawbones stronger and more effective on
this, the eyeless side of the head, than on the other, for the sake, as Dr.
Traquair supposes, of feeding with ease on the ground. Disuse, on the
other hand, will account for the less developed condition of the whole in-
ferior half of the body, including the lateral fins; though Yarrel thinks
that the reduced size of these fins is advantageous to the fish, as "there
is so much less room for their action than with the larger fins above."
Perhaps the lesser number of teeth in the proportion of four to seven in
the upper halves of the two jaws of the plaice, to twenty-five to thirty
in the lower halves, may likewise be accounted for by disuse. From the
colorless state of the ventral surface of most fishes and of many other
animals, we may reasonably suppose that the absence of color in flat-fish
on the side, whether it be the right or left, which is undermost, is due to
the exclusion of light. But it cannot be supposed that the peculiar speckled
appearance of the upper side of the sole, so like the sandy bed of the sea,
or the power in some species, as recently shown by Pouchet, of changing
their color in accordance with the surrounding surface, or the presence
of bony tubercles on the upper side of the turbot, are due to the action
of the light. Here natural selection has probably come into play, as well

Online LibraryCharles DarwinThe origin of species → online text (page 21 of 50)